Constant Pain in Right Thumb and Palm?

I have been having pain in my right thumb for the last 2 years. The only way I can describe it is a constant soreness that runs from the tip of the thumb into the palm. I have been to numerous doctors including neurologists, orthopedists hand surgeons and chiropractors. Tests done so far include 2 nerve conduction's, nerve block, mri on thumb, wrist and spinal cord, x-ray's on hand back and even a ultrasound - all negative. No one has been able to diagnose what the problem is. They have told me it is not carpel tunnel syndrome. Is there anything else that could be causing a pain like this?